Commercial multifocal lenses come in a variety of materials and are generally made of plastic or glass. These lenses come in many styles, sizes, and can be of a lined, blended or progressive design. Of these designs, lined bifocals have long been used by those requiring near vision correction. The lined bifocal segment is fused in the case of glass, or molded in the case of plastic. In either case, the bifocal segment line is noticeable and represents the junction of near and distance optical portions in the lens or a semi finished blank providing the distance focus and near focus. Bugbee (U.S. Pat. No. 1,509,636), Meyrowitz (U.S. Pat. No. 1,445,227), and Culver (U.S. Pat. No. 2,053,551) teach fused lined bifocals or multifocals. While lined bifocals have been used successfully for many years, they have several drawbacks. First, they are extremely noticeable and thus are not cosmetically appealing; second, the segment line creates a blur when looking from far to near objects and vice versa; and third, there is an abrupt change of focal length when looking from far to near objects and back again. No optical area is provided with intermediate power (focal length) at all, unless a lined trifocal is used.
Blended bifocals, such as those disclosed in W082/03129, are bifocals which retain a clear demarcation between the optical zones with far focus and near focus; however, the line of demarcation is blended to make it far less noticeable. Blended bifocals attempt to solve the cosmetic disadvantage of a lined bifocal, but in doing so create a wide blended blur zone when looking from far to near objects and back again, as well as failing to provide intermediate vision.
Progressive addition lenses are a type of multifocal lens which incorporate a progression of power changes from far to near correction, creating a progressive vision transition from far to near power and back again. Progressive addition lenses represent an attempt to solve the problems discussed above. Although progressives solve several deficiencies of lined or blended bifocal lenses, they require other compromises in optical design, which in turn compromise the visual function of the lens optic, as discussed below. Progressive addition lenses are invisible, and provide a natural transition of power from far to near foci.
Methods of making progressive addition lenses are disclosed, for example, by Harsigny (U.S. Pat. No. 5,488,442), Maitenaz (U.S. Pat. No. 4,253,747), Maitenaz (U.S. Pat. No. 3,687,528), Cretin et al. (U.S. Pat. No. 3,785,725), Maitenaz (U.S. Pat. No. 3,910,691), Winthrop (U.S. Pat. No. 4,055,379), Winthrop (U.S. Pat. No. 4,056,311), and Winthrop (U.S. Pat. No. 4,062,629). These lenses, however have certain deficiencies which are inherent in their design. A first deficiency is that only a relatively narrow reading channel width of about 3-8 mm, defined as the space between two meridional imaginary lines characterized by astigmatism of +/-0.50 diopters or more. This reading channel represents the progressive transition of focal lengths from far to near, enabling one to see from far to near in a somewhat natural manner without experiencing the abrupt change of power of a lined bifocal. A second deficiency is that progressive addition lenses can only provide a relatively narrow reading zone which is about 22 mm wide or less. A third major deficiency is the unwanted peripheral astigmatism which is created due to the nature of the progressive optical design. This unwanted peripheral astigmatism creates significant visual distortions for the user. Manufacturers are interested in limiting the amount of unwanted astigmatism in order to enhance the visual performance, and thus increase the acceptance levels of their different designs. In practice, all progressive lens designs represent compromises among having a lens with the widest possible channel, the lowest amount of unwanted astigmatism and the widest add power zone. A fourth major deficiency is the difficulty of properly fitting the patient with a progressive; and, the fifth deficiency is the low tolerance for fitting error allowed by these designs.
Numerous attempts have been made to solve the inherent problems discussed above with lined, blended, trifocals, and progressive multifocals. However, no other commercially viable options have been found. The ophthalmic lens design disclosed in Frieder (U.S. Pat. No. 4,952,048) and Frieder (U.S. Pat. No. 4,869,588) addresses some of these deficiencies, but fails to provide a satisfactory solution due to both manufacturing difficulties and poor cosmetic appearance at moderate to higher add powers. Although these patents disclose a lens with several improved features, in the moderate to higher add powers from +1.75 to +3.00 diopters, this lens caused the front (convex) surface defining the periphery of the near power zone to bulge anteriorly and thus cause a visible optical distortion on either side of the reading zone. This feature significantly reduced its commercial appeal. Furthermore, difficulties in manufacturing this lens made the lens less commercially viable.
Maeda (U.S. Pat. No. 4,944,584) discloses a refractive gradient lens using a first partially cured substrate layer. A second uncured resin layer is added and diffusion occurs between these two layers during curing to create a third diffusion layer having a refractive index gradient which varies continuously between the refractive indices of the first and second layers. To achieve this diffusion layer, the assembly containing the second layer is heated at specified temperatures for 20-26 hours. The time required for curing to form the diffusion layer makes this procedure unattractive from a commercial standpoint. Furthermore, it is known that the process disclosed in Maeda, which includes demolding a partially cured lens or semi-finished blank, can create yield problems. Thus, while it may be theoretically possible to achieve Maeda's third, continuously varying, refractive index gradient diffusion layer, actual manufacturing difficulties may reduce the likelihood that the Maeda lens could achieve commercial success.
In addition to the deficiencies previously mentioned concerning bifocal and multifocal lenses, these lens styles are also thicker than single vision lenses of equivalent distance power, since they are required to provide for additional plus power in the add power zone. This added thickness on the anterior surface of the lens tends to detract from their cosmetic appeal and adds additional weight to the lens. Several solutions to this problem have been proposed.
Blum (U.S. Pat. No. 4,873,029) describes the use of a preformed wafer having desired multifocal segments formed thereon and adding a resin layer of a different index of refraction onto the surface of the preformed wafer. In this approach, the preformed wafer is consumed during the molding process so that the preformed wafer ultimately forms part of the lens. While this approach produces a cosmetically improved lens, the process requires hundreds of gaskets and back convex spherical and toric molds. These molds ultimately make the concave side of the finished lens. Furthermore, with this approach the bifocal or multifocal zone is not invisible due to the significant refractive mismatch needed and the lack of a transition of refractive indices of various materials.
Various patents disclose refractive gradient bifocal, multifocal or progressive lens styles, e.g., Dasher (U.S. Pat. No. 5,223,862), Maeda (U.S. Pat. No. 4,944,584), Yean (U.S. Pat. No. 5,258,144), Naujokas (U.S. Pat. No. 3,485,556), Okano (U.S. Pat. No. 5,305,028), Young (U.S. Pat. No. 3,878,866), Hensler (U.S. Pat. No. 3,542,535), and Blum (U.S. Pat. No. 4,919,850). However, the commercial production of refractive gradient multifocal ophthalmic lenses to date has not been commercially successful due to chemistry, technology, manufacturing and cost limitations.
In European Patent Application No. PCT/US93/02470, Soane discloses producing a multifocal lens having a bifocal and astigmatic area on the back, concave side of the front optical wafer preform. Soane discloses curing a resin material having a different index of refraction from the optical wafer preform onto the back of the front optical wafer preform using an appropriate back convex mold having the correct curvature. This approach, however, requires that a significant number of front optical preforms be inventoried.
In view of the above, it is desirable to have a progressive multifocal lens which would allow the end user a wide and natural progression of vision when looking from far to near, being substantially free of or having a reduced amount of unwanted peripheral astigmatism, having a wide reading zone, requiring a smaller inventory of skus (stock keeping units) and being relatively forgiving and easy to fit for the patient. In addition, it would be desirable to have a progressive multifocal lens which has substantially the same thickness as a single vision lens of equivalent distance prescription, and which cosmetically is almost invisible in appearance. Also, it is desirable to manufacture such optical products in a way that reduces the amount of processing time.